A number of biomedical devices such as Electrocardiography (ECG), Electroencephalography (EEG), Electromyography (EMG), etc. require amplification of a number of bio-signals sensed from a number of electrodes attached to a patient. Usually, such equipment uses repeated stages of identical analog amplifier circuitry to amplify each of the signals sensed by the electrodes. This results in increased hardware, power consumption, form factor and cost. For example, a typical 12-Lead ECG system requires eight identical analog amplifier channels for amplifying signals from a combination of 10 electrodes attached to a patient. The remaining four ECG signals are computed mathematically from the eight amplified signals. Each ECG amplifier channel is implemented with a number of analog components.
Usually, such amplifiers need a band pass frequency response from near direct current DC to several hundred hertz. For example, a diagnostic ECG amplifier requires a frequency response from 0.05 Hz to 150 Hz. In order to get 0.05 Hz high-pass frequency response, it is necessary to use AC coupled amplifier stages with large time constants, which have long settling times (in several seconds). Since the settling time of the AC coupled (high-pass filter (HPF)) stage of such an amplifier channel is quite high, it is not possible to multiplex a single analog amplifier channel to amplify multiple signals.
In the conventional ECG, each of the eight ECG channels is implemented with individual differential amplifiers, high-pass filters, gain amplifiers and anti-aliasing filters. If N equals the number of identical amplifier channels (N=8 in case of 12-Lead ECG), C equals the number of electronic components per channel and F equals the components required for the RL-drive circuit and input buffers, then the total number of electronic components needed to implement a conventional 12-Lead amplifier system equals (N×C)+F. The total number of components in identical amplifier channels, taken together, is much higher than the components in RL drive and input buffers. Hence, N×C>>F. Increased numbers of components increases the printed circuit board complexity, the cost, the form factor and results in higher power consumption.
Further, in the conventional ECG, aging of analog components and environmental factors such as temperature variation can cause drifts and artifacts in the measured signals. Since each ECG channel is physically separate, each ECG channel is subject to a different drift due to environmental and component variations.